Addressable photochromic memory display device

ABSTRACT

An addressable photochromic memory display device comprising a readout area of a photochromic material, a means for uniformly exposing the photochromic material to electromagnetic radiation whereby the photochromic material changes color, a means for reversing the color change in selected areas of the photochromic material whereby a readout is formed in the readout area.

Edited States Patent [191 Chang et al.

[ ADDRESSABLE PHOTOCIIROMIC MEMORY DISPLAY DEVICE [75] Inventors: Ifay F. Chang, Mahopac; Russell W.

Dreyfus, Cross River, both of NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Dec. 12, 1972 [21] Appl. No.: 314,290

52 us. Cl. 250/461 [51] Int. Cl. G0ln 21/38 [58] Field of Search 250/461 [56] References Cited I UNITED STATES PATENTS ,327,120 6 1 67 Weiss;.,...gj.I l,I;fiig nl'lffib'l i'l" m 3,813,554 1 May 28, 1974 3,428,396 2/1969 Megla et al. 250/46l X 3,541,330 11/1970 Eichelberger et al... 250/4 61 X 3,676,677 7/1972 Condas ct al. 250/461 X Primary Exami ner-Archie R. Borchelt I Attorney, Agent, or Firm-Sughrue, Rothwell, Mion, Zinn & Macpeak 57 ABSTRACT An addressable photochromic memory display device comprising a readout area of a photochromic material, a means for uniformly exposing the photochromic material to electromagnetic radiation whereby the photochromic material changes color, a means for reversing the color change in selected areas of the photochromic material whereby a readout is formed in the readout area.

27 Claims, 3 Drawing Figures ADDRESSABLE PHOTOCIIROMIC MEMORY DISPLAY DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an addressable display device. More particularly, this invention relates to an addressable display device in which a photochromic compound is employed.

2. Description of the Prior Art Many different approaches to addressable display devices are known. Presently known displays use NIXIE tubes, an array of light emitting devices, a liquid crystal display and tungsten filament approaches to provide display devices which can be selectively addressed. Some of the criteria which determine which of these approaches are used in addressable display devices are low cost and manufacturing simplicity,low power consumption during operation, long operating life with minimal maintenance, high fatigue resistance and the speed with which the display on the addressable device can be erased or altered. A number of approaches to accomplish these objects are known in the prior art.

More particularly, in US. Pat. No. 3,529,155, a display device is disclosed in which a material, such as zinc-cadmium-sulfide, containing silver and nickel, which fluoresces when continually subjected to ultraviolet light, is activated using ultraviolet light and a readout is provided in this fluorescent readout surface by application of heat in selected areas of the readout. The heat applied in the selected areas to form the readout quenches the fluorescence in those areas. This luminescent display device has the disadvantage that it must be operated in a manner in which ultraviolet light is continuously applied to the readout surface to cause the material to fluoresce and heat must be continually supplied to the selected pattern areas in order to quench the fluorescence in those pattern areas to form the readout. Thus, this display device requires the consumption of power continuously during operation.

In US. Pat. No. 3,323,241,21 thermochromic display device is disclosed. As disclosed therein, a layer of a thermochromic material such as copper or mercury halides is deposited over an array of heat generating elements. The thermochromic material employed changes color upon the application of heat. When this material is used in the thermochromic display device disclosed in this patent, the heat generating elements adjacent the thermochromic material are actuated causing the thermochromic material in selected areas to change in color forming a visible character in accordance with the pattern applied in actuating the heat generating elements. Again, as is the situation previously described with respect to US. Pat. No. 3,529,155, it is necessary that the heating elements operate continuously in order out area but also power consumption to quench for pattern formation. Accordingly, there is no memory characteristic associated with the stimulus supplied, i.e., the heat or light causing fluorescence or color change. and

once the stimulus is removedthe displayed pattern disappears.

It is therefore an object of this invention to provide a display device having memory characteristics.

It is also an object of this invention to provide a display device in which, once the display device is addressed, no power consumption is required to retain the pattern displayed.

It is also an object of this invention to provide an addressable display device which can be manufactured easily and simply as well as to provide a display device which has excellent responsetimes, low fatigue characteristics, and minimal power consumption for operation.

SUMMARY OF THE INVENTION These and the other objects of this invention are accomplished by the invention described herein.

This invention comprises an addressable photochromic memory display device which comprises a readout area of a thermochromic and photochromic material, a means for uniformly exposing the thermochromic and photochromic material to electromagnetic radiation of an appropriate wavelength causing the thermochromic and photochromic material to change color, and a means for subsequently selectively reversing this color change in the thermochromic and photochromic material so that a pattern is formed in the readout area. Hereafter, the term photochromicf' will be used at times for brevity and is intended to cover those materials which are photochromic and thermochromic.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIG. I shows an embodiment of the addressable thermochromic andphotochromic memory display device of this invention comprising a readout area of a thermochromic and photochromic material coated on an array of selectively addressable heating elements and an ultraviolet light source for uniformly activating the thermochromic and photochromic material.

FIG. 2 shows another embodiment of the addressable thermochromic and photochromic memory display device of this invention comprising a readout area of a thermochromic and photochromic material, an electron beam source for uniformly exposing the thermochromic and photochromic readout area for activation of the photochromic material and an array of light emitting devices which can be appropriately addressed to selectively reverse the color change in the photochromic material to form a pattern or display in the readout area.

FIG. 3 shows an additional embodiment of the addressable thermochromic and photochromic memory display device of this invention comprising an electron beam source for uniformly exposing the readout area for activation of the photochromic material and a source of laser light for scanning and selectively reversing the color change to form a display in the readout area.

DETAILED DESCRIPTION OF THE INVENTION As was described above, the addressable photochromic and thermochromic memory display device of this invention comprises the use of a thermochromic and photochromic material, a means for activating the photochromism of the material to cause the material to change color and a second means for selectively reversing this color change in certain areas to form a pattern in the readout area.

Photochromic materials are known in the art and due to their photochromism they change color, e.g., from colorless to colored or from one color to a second color, when subjected to an appropriate stimulus. For example, the photochromism can be exhibited by subjecting the photochromic material to excitation by bathing the photochromic material in ultraviolet light. Once the color change has'occurred due'to the excitation, the photochromic material retains or remembers this color until such time as a second stimulus is applied to the material or after an elapse'of a long period of time due to decay from the metastable state. For all practical purposes the decay time is infinite with respect to the time required to generate a display. For example. the change in color, for example, from colorless to colored or from a first color to a second color, can be reversed, i.e., from colored to colorless or from the second color to the first color, upon the application of light of a different wavelength from that used to excite the photochromic material initially or by the application of heat. I

Suitable photochromic materials which can be employed in the addressable display device of this invention are materials such as photochromic sodalite, such as Na,,Al,,Si O :2NaCl(S), as disclosed in Williams et al., J. Am. Ceramic $00., 52 139 (I969), doped titanates such as SrTiO (Fe,Mo) as disclosed in I.E.E.E. Jour. of Quantum Electronics, QES, page 17 (1969), and U.S.Pats. Nos. 3,329,648 and 3,322,552, or doped fluorides such as CaF (La) or CaF (Eu, Sm), as disclosed in U.S. Pat. No. 3,452,332 and Applied Solid State Science, 2, 107 (1971) by B. W. Faughnan, D. L. Staebler and Z. J. Kiss. By employing these materials which are both photochromic and thermochromic, it is possible to activate them with anappropriate stimulus to cause the color change discussed above and then to deactivate the color in selected areas whereby the normal color of the photochromic material in the equilibrium state returns forming a display pattern in the readout area of the display device due to the contrast between the deactivated areas and the excited colored areas. For example, in selected areas light of an appropriate wavelength or heat can be applied to the readout area to form the pattern to be displayed.

By using the memory characteristics of the photochromic material once the photochromic'material is excited uniformly and selectively deactivated to form k a pattern, the display device can be utilized with no additional power consumption as is required in the prior art devices discussed above. Alternatively, the display device of this invention employing the thermochromic and photochromic material can be activated for use and the material remains in this activated state with no additional power consumption being utilized to maintain this material in the activated state. The display device thus is in a condition that at some time subsequent to the activation, a pattern can be formed on the readout area of the display device by selectively applying a stimulus which will cause selected areas to return to the normal state. Similarly, with the addressable photochromic memory display device of this invention, a pattern can be grown on the readout area due to the memory characteristics of the thermochromic and photochromic material employed. For example, the photo- 'Photochromic Material chromic material can be excited, selected areas of the readout deactivated to form a pattern in accordance with a predetermined signal to form a pattern, and the pattern expanded or selectively (or partially) erased by sequentially deactivating additional excited areas of the photochromic device.

As was described above, an appropriate stimulus for exciting the photochromic material and deactivating the photochromic material is necessary to provide the readout in the readout area for the addressable photochromic memory display device of this invention. The following table shows such appropriate stimuli for suitable photochromic materials which can be used in this invention.

Excitation Deactivation Heating to I00C 3. Rare earth dope'd fluorides Exposure to UV light of 2000 A to 4000 A Exposure to light centered around One embodiment of the addressable display device of this invention is shown in FIG. 1. In FIG. 1, l is a thermally insulating support; 2 represents an array of heating elements; 3 represents a coating of a photochromic material over the heating elements 2 and the thermally insulating support 1 to form readout area 4; 5 represents lead-ins to the heating elements which are connected to a source of electric current, not shown, for selectively activating the heating elements; 6 represents an ultraviolet light source which uniformly bathes the readout area with light of an appropriate wavelength to excite and to cause the layer of photochromic material 3 to change color.

In the operation of the addressable thermochromic and photochromic memory display device of this embodiment, the readout area 4 comprising a coating of, for example, sulfochlorosodalite, over the heating elements and the support is uniformly exposed to ultraviolet light from ultraviolet light source 6 whereby the photochromic layer 3 changes color from white to purple.'After this uniform exposure, a selected set of heating elements 2 is actuated by passing an electric current through lead-in connectors 5 whereby heat is generated by the heating elements so actuated and which are adjacent to the photochromic layer 3. The effect of the heat generated in the areas adjacent those heating elements actuated causes a reversion in the color change of the photochromic material 3 from the excited state exhibiting a purple color to the normal state having a white color whereby a pattern is formed due to the differences in these colors. In the above embodiment of the addressable display device of this invention, the heating resistance'elements 2 can beactivated through lead-ins 5 by any of the selective means known in the art, such as a matrix address scheme.

In the above embodiment of the display device of this invention, it can be seen that only initial excitation with UV light and subsequent activation of the heating elements to form the pattern in the readout area is required. Once the power requirements to accomplish the initial excitation and the subsequent pattern formation on the readout area of the addressable display device of this invention are expended, the layer of the thermochromic and photochromic material, due to its memory or storage characteristics, continually displays the information to be exhibited without the need for any additional power consumption.

Another embodiment of the addressable display device of this invention having the memory characteristics discussed is shown in FIG. 2 in which is a thermally insulating support, which can be transparent or translucent, having coated thereon a layer of a photochromic material 11; I2 are electron beam sources for uniformly irradiating the photochromic material II in an evacuated envelope 13 with a flood of electron beams to excite the photochromic material in the photochromic layer ll into its colored state. Reference numeral l4 designates an array of light emitting devices which can be selectively addressed through lead-ins 15 which, by appropriately energizing the lead-ins using a matrix address technique the light emitting devices 14 can be selectively activated to emit light of an appropriate wavelength to reverse the color change" generated by the flood of electron beams, for example, light emitting diodes of Gal? peaking at approximately 5,500 A can be used to form the pattern on the colored photochromic layer where sulfobromosodalite is used as the photochromic layer ll. This wavelength of light is appropriate for bleaching selected areas of the photochromic layer to cause a pattern display to occur in the readout area. Where other photochromic materials are employed, light emitting diodes peaking at an appropriate wavelength which can be determined simply by scanning the colored photochromic material with lights of varying wavelengths to determine the optimum wavelength of light for reversing the color change and bleaching out the color formed due to theexcitation of the photochromic layer. An array of these light emitting devices 14 which can be selectively addressed in accordance with a predetermined pattern or display can be actuated appropriately using a matrix address or line scan in this second embodiment of the addressable memory display device of this invention.

In a third embodiment as shown in FIG. 3, a laser scanning beam from laser 25 can be used in place ofthe array of light emitting devices as shown in FIG. 2. In FIG. 3, is a thermally insulating support, which can be transparent or translucent, having coated thereon a layer of a photochromic material 21; 22 are electron beam sources for uniformly irradiating the photochromic material 21 in an evacuated envelope 23 with a flood of electron beams to excite the photochromic material in the photochromic layer 2i into its colored state. 24 is a rotating mirror for directing the laser light onto the photochromic material 21; to scan and deactivate selected portions of the photochromic material 21.

In addition to the above embodiments, other excitation sources for uniformly exciting the photochromic layer on the support of the readout area can be employed such as plasma (gasdischarge).

In addition to the above, other means for reversing or deactivating the color generated upon excitation into the excited state can be employed. In addition to the above described heating means and light emitting device array, a light emitting film or filaments, a laser array such as semiconductor laser or an electroluminescent (ZnS) film in place of the heating elements or LED array can be used.- Basically, all that is needed in actuating and forming the display pattern on the readout of the addressable memory display device of this invention is, subsequent to the overall excitation of the photochromic material to cause a generation of color, to selectively reverse this color change by deactivating the color using a stimulus appropriate to the photochromic material employed in the readout area.,One skilled in the art can determine the appropriate stim ulus by simply exposing the photochromic material, in the excited state, to electromagnetic radiation and determining the optimum source of electromagnetic radiation and wavelength for causing the appropriate bleaching to occur.

In all of the addressable memory photochromic display devices of this invention, the memory effect of the photochromic material employed in the readout area results in these devices being essentially a zero power consumption display device, the only power being consumed is that required to cause the initial excitation, e.g., to cause the initial color change, the deactivation in selected areas to cause a reversal of this color change which, due to the contrast, results in the formation of a pattern, and where appropriate, the switching of these states to form different or new patterns in.the readout area, for example, complete erasure and new pattern formation. In contrast to the prior art,fluores-.

cent display devices, as discussed above, neither the stimulus for exciting the photochromic layer nor the stimulus used for addressing the pattern on the readout area needs to be maintained constantly. In the prior art 7 devices, appropriate excitation was necessary to cause fluorescence. In contrast 'to the prior art devices, the display device of this invention possesses a memory which results in power being consumed only at the times of change in the display or erasure of the display and a subsequent addressing of anew display in the readout area.

Suitable photochromic materials which can be employed forcoating the readout area in the addressable memory photochromic display device of this invention are photochromic sodalite which can contain different alkali metal halides and activators such as sulfur or iron to give rise upon excitation of pink, purple, magenta, blue or black colors. For example, a photochromic sodalite containing sodium iodide as a dopant material upon-excitation will give rise to a blue color, a photochromic sodalite containing sodium chloride as a dopant material will, upon excitation, generate a purple color, a photochromic sodalite containing potassium iodide as the dopant material will give rise to a black color. Thus, depending upon the ambient light characteristics within which the addressable photochromic memory display device of this invention is to be employed, a photochromic material having the appropriate contrast characteristics can be selected for optimal visual observation. Other photochromic materials having the memory characteristics discussed above can be doped titanates such as SrTiO (Fe, M0) or doped fluorides such as CaF (La).

In addition to the above described advantages of zero power consumption except when switching states, the addressable memory photochromic display device of this invention can be actuated rapidly, e.g., a few seconds are only required for addressing the pattern on the causing color reversal may be arranged in a matrix array and a pattern is then formed by selecting, through addressing, the X and Y coordinates of the particular elements forming the pattern. When a more rapid change in the display is desired, a half select mode of operationcan be utilized. In the following discussionof the half select mode, the use of heating elements will be employed for the purposes of illustration of this half select mode. In greater detail, the coordinates of the selected heating elements along one axis of the matrix, e.g., the X axis, are energized. This preliminary energization or increase in temperature is, however, insufficient to increase the temperature to the point where the photochromic material undergoes a color reversal. The required temperature for color reversal is a relatively sharply defined quantity since the color change occurs rapidly depending exponentially on the reciprocal of the temperature. When a colorreversal is desired, the coordinates of the selected elements on the other axis, e.g., in this example the Y axis, are then energized or simultaneously energized. The additional energy provides a sufficient stimulus to cause the color reversal. In the X-Y matrix address scheme, the display can be actuated oneline at a time, for example, by a scanning method. By using this half-select method", the speed with which pattern formation can be accomplished can be markedly increased.

From the description above, the addressable photochromic memory display device of this invention has obvious advantages not achievable with prior art devices. lt'will also be apparent that various changes and modifications can be made in the embodiments specifically'described above without departing from the spirit and scope of this invention.

What is claimed is:

1. An addressable photochromic memory display device comprising:

a readout area of a photochromic material comprising a support and a layer of a photochromic material thereon, said layer comprising the readoutyielding elements of said readout area;

a means for uniformly exposing said photochromic material to appropriate electromagnetic radiation, whereby said photochromic material changes color; and

a means for reversing said color change in said photochromic material in selected areas of said readout area, whereby a readout is formed in said readout area, said means for reversing said color change comprising an addressable matrix array of energy emitting elements which cause said color change in selective areas in said photochromic material.

2. The display device of claim 1, wherein said means for uniformly exposing said photochromic material comprises an ultraviolet light source.

3. The display device of claim I, wherein said means for uniformly exposing said photochromic material is an electron beam source.

4. The display device of claim 1, wherein said means for reversing said color change comprises an array of heating elements.

5. The display device of claim 1, wherein said means for reversing said color change comprises an array of light emitting devices.

6. The display device of claim 1, wherein said means for reversing the color change comprises a laser array.

7. The display device of claim 1, wherein said photochromic material is photochromic sodalite, a transition metal doped strontium titanate, or a rare earth doped calcium fluoride.

8. The display device of claim 7, further including means for energizing said matrix array in a half select mode. v

9. The display device of claim 1, wherein said means for reversing said color change comprises an X-Y addressable'matrix.

10. The display device of claim 9, wherein said matrix comprises heating elements.-

11. The display device of claim 10, wherein saidmeans for uniformly exposing said photochromic material comprises an ultraviolet light source.

12. The display device of claim 9, wherein said support is thermally insulating.

13. The display device of claim 12, wherein said support is transparent.

14. The display device of claim 12, wherein said support is translucent.

15. The display device of claim 12, further including means for energizing said matrix array in a half-select mode. r

16. The display device of claim 9, wherein said matrix comprises light emitting devices.

17. The display device of claim 16, wherein said means for uniformly exposing said photochromic material is an electron beam source.

18. The display device of claim 16, wherein said support is thermally insulating.

19. The display device. of claim 18, wherein said support is transparent.

20. The display device of claim 18, wherein said support is translucent.

21. The display device of claim 18, further including means for energizing said matrix array in a half-select mode.

22. The display device of claim 9, wherein said matrix comprises heating elements directly coated in layer form on said support, and said photochromic material is directly coated upon said heating elements.

23. The display device of claim 9, wherein said matrix comprises light emitting devices, said photochromic material being coated in layer form on said support and said photochromic material being in direct contact with said light emitting devices at the surface of said photochromic material opposite the support.

24. The display device of claim 1, wherein said photochromic material which has undergone said reversal in color change in selected areas maintains said reversed color change without additional power consumption.

25. The display device of claim 6, wherein said readout area and said means for reversing said color change in selected areas are in direct physical contact.

26. The display device of claim 1, wherein said readout area and said means for reversing said color change in selected areas are separated.

27. The display device of claim 9, further comprising means to address said matrix.

* i i I. 

1. An addressable photochromic memory display device comprising: a readout area of a photochromic material comprising a support and a layer of a photochromic material thereon, said layer comprising the readout-yielding elements of said readout area; a means for uniformly exposing said photochromic material to appropriate electromagnetic radiation, whereby said photochromiC material changes color; and a means for reversing said color change in said photochromic material in selected areas of said readout area, whereby a readout is formed in said readout area, said means for reversing said color change comprising an addressable matrix array of energy emitting elements which cause said color change in selective areas in said photochromic material.
 2. The display device of claim 1, wherein said means for uniformly exposing said photochromic material comprises an ultraviolet light source.
 3. The display device of claim 1, wherein said means for uniformly exposing said photochromic material is an electron beam source.
 4. The display device of claim 1, wherein said means for reversing said color change comprises an array of heating elements.
 5. The display device of claim 1, wherein said means for reversing said color change comprises an array of light emitting devices.
 6. The display device of claim 1, wherein said means for reversing the color change comprises a laser array.
 7. The display device of claim 1, wherein said photochromic material is photochromic sodalite, a transition metal doped strontium titanate, or a rare earth doped calcium fluoride.
 8. The display device of claim 7, further including means for energizing said matrix array in a half select mode.
 9. The display device of claim 1, wherein said means for reversing said color change comprises an X-Y addressable matrix.
 10. The display device of claim 9, wherein said matrix comprises heating elements.
 11. The display device of claim 10, wherein said means for uniformly exposing said photochromic material comprises an ultraviolet light source.
 12. The display device of claim 9, wherein said support is thermally insulating.
 13. The display device of claim 12, wherein said support is transparent.
 14. The display device of claim 12, wherein said support is translucent.
 15. The display device of claim 12, further including means for energizing said matrix array in a half-select mode.
 16. The display device of claim 9, wherein said matrix comprises light emitting devices.
 17. The display device of claim 16, wherein said means for uniformly exposing said photochromic material is an electron beam source.
 18. The display device of claim 16, wherein said support is thermally insulating.
 19. The display device of claim 18, wherein said support is transparent.
 20. The display device of claim 18, wherein said support is translucent.
 21. The display device of claim 18, further including means for energizing said matrix array in a half-select mode.
 22. The display device of claim 9, wherein said matrix comprises heating elements directly coated in layer form on said support, and said photochromic material is directly coated upon said heating elements.
 23. The display device of claim 9, wherein said matrix comprises light emitting devices, said photochromic material being coated in layer form on said support and said photochromic material being in direct contact with said light emitting devices at the surface of said photochromic material opposite the support.
 24. The display device of claim 1, wherein said photochromic material which has undergone said reversal in color change in selected areas maintains said reversed color change without additional power consumption.
 25. The display device of claim 6, wherein said readout area and said means for reversing said color change in selected areas are in direct physical contact.
 26. The display device of claim 1, wherein said readout area and said means for reversing said color change in selected areas are separated.
 27. The display device of claim 9, further comprising means to address said matrix. 